Clamping devices of this type are used in orthopedics and traumatology for various purposes as components of internal or external fixation devices. For example, Swiss Patent Application 00186/96 describes an internal fixation device which is used in case of morbid changes, such as spondylosis at vertebrae and disks, wherein the position of the vertebrae is corrected and the relative position of the affected vertebrae caused by the correction is fixed in place.
A fixation device of the type mentioned at the outset basically consists of an implant pin which is fastened in a bone. Generally pins with a thread, i.e. screws, are used as implant pins, which are screwed into a prepared bore in the bone. The portion of the implant pin outside of the bone protrudes through a first recess, or respectively bore of a clamping body. Within the scope of the instant description, the term "clamping arrangement" should be understood as being the clamping body and the implant pin fastened therein. The clamping body has a second bore which is oriented transversely in respect to the first bore and in which a clamping screw is received. A slit starting at the first bore and extending perpendicularly in respect to the longitudinal axis of the clamping screw forms two resilient clamping jaws on the clamping body. The slit is arranged in such a way that its imagined extension contains the longitudinal axis of the implant pin. The clamping screw is used for clamping the clamping jaws together and in the process to fix the implant pin in place in the clamping body. The head of the clamping screw is designed in such a way that it can receive a connecting rod, which also extends through respective heads of clamping screws of one or several more fixation devices, wherein the connecting rod can be straight or curved, depending on the requirements. With internal fixation devices, all above described components are inside the body of the person with the implant, while with external fixation devices only a portion of the implant pin is arranged in the body of the person with the implant, while the remainder of the implant pin protrudes out of the body, so that the clamping body and the connecting rod are located outside the body of the person with the implant.
In the course of movements of the person with the implant, the implant pin and the clamping body also move, which can also cause relative movements between the implant pin and the clamping body. Regardless of whether these are linear movements in the direction of the axis of the implant pin or rotations of the implant pin or bending of the implant pin, these relative movements are always minute movements. The forces transferred in the course of the relative movements between the implant pin and the clamping body not only result in mechanical wear indications on the surface, caused by friction, but mainly also, since these are alternating stresses, in fatigue of the material, which has a negative effect on the service life of the clamping arrangement. This undesired effect not only occurs in the cross sections where the implant pin exits the clamping body, but at the entire contact surface between the implant pin and the clamping body. Material fatigue in particular results in the formation of fine cracks, which has a particularly disadvantageous effect on the implant pin as the mechanically weaker component. If stress peaks occur over time because of particular movements of the person with the implant, larger cracks extending from the fatigue cracks can form, from which overload breakage can develop in the end. As already mentioned, it is also disadvantageous that rubbed off particles are created from the frictional forces between the surface of the implant pin and the wall of the recess in the clamping body, which are also a sign of undesirable wear and which, in the case of internal fixation devices, can lead to metallosis in the case of internal fixation devices. Although generally implants are made of materials which are physiologically well tolerated, this applies only to particles of macroscopic size, while microscopic elements, such as rubbed-off particles made of the same materials are physiologically not tolerated and can result in the metallosis mentioned.